Semiconductor light-emitting device and method of forming electrode

ABSTRACT

A semiconductor light-emitting device having an electrode that can be manufactured by a simple method and is unlikely to deteriorate, and a method for forming the electrode are provided. The semiconductor light-emitting device according to the present invention has a semiconductor layered structure having a light-emitting layer that emits light by supplying electric power and an electrode formed on the semiconductor layered structure. The electrode has a reflection layer that reflects light exiting from the light-emitting layer, a barrier layer formed on the upper side and side surface of the reflection layer, and a pad layer formed only on the top surface of the barrier layer.

CROSS REFERENCE TO RELATED APPLICATION

This Nonprovisional application claims priority under 35 U.S.C. §119(a)on Patent Applications No. 2012-004669 filed in Japan on Jan. 13, 2012and No. 2012-202199 filed in Japan on Sep. 14, 2012 the entire contentsof which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor light-emitting devicerepresented by light-emitting diode (LED) and the like, and a method forforming an electrode provided in the semiconductor light-emittingdevice.

2. Description of the Related Art

The semiconductor light-emitting device represented by LED and the likehas advantages such as low power consumption, compact in size, highbrightness, and long life, and thus is recently used for variousapplications. For example, the semiconductor light-emitting device hasbeen used for a lighting system, as a substitute for incandescent lampthat consumes a large amount of electric power.

In addition, a semiconductor light-emitting device having a reflectionlayer capable of reflecting light exiting from a light-emitting layer inan electrode, in order to further increasing light extractionefficiency, is suggested. For example, in Japanese Unexamined PatentPublication No. 2002-26392, Japanese Unexamined Patent Publication No.2008-41866, Japanese Unexamined Patent Publication No. 2011-66461,Japanese Unexamined Patent Publication No. 2006-80469, JapaneseUnexamined Patent Publication No. 2006-93358, and Japanese UnexaminedPatent Publication No. 2011-204804 (hereinafter, referred to as“publicly known documents 1 to 6”), a semiconductor light-emittingdevice using Al that has high reflectance and is inexpensive as areflection layer is suggested.

However, Al has a low melting point and is a chemically active material,therefore, there are problems that the surface is susceptible tocorrosion, and void and hillock, and migration are likely to occur inthe interior. Also, Al is a material that is likely to interdiffuse whencontacting with a dissimilar metal, and for example, when contactingwith Au, an intermetallic compound of AuAl that is highly resistive andfragile is formed by interdiffusion, and it becomes a factor to damagethe reliability of the semiconductor light-emitting device, such thatthe forward voltage of the semiconductor light-emitting deviceincreases, the adhesion strength of the electrode deteriorates, or thelike.

Au is often used as a wire connecting an external power supply and asemiconductor light-emitting device, and in order to favorably connectto this wire, Au is often used as a pad layer of the electrode. However,when both Al layer and Au layer are contained in the electrode, theproblem of contact described above arises.

Therefore, for example, publicly known documents 1 to 6 suggest asemiconductor light-emitting device having an electrode in which directcontact between an Al layer and an Au layer is avoided by insertingbetween the Al layer and the Au layer a layer made of a material otherthan Al and Au.

Publicly known documents 1 to 5 suggest a semiconductor light-emittingdevice having an electrode with a structure in which a layer made of amaterial such as Ti, Pt, W, Ta, and Pd is inserted between the Al layerand the Au layer. However, in the electrodes of the semiconductorlight-emitting devices described above, the side surface of the Al layeris exposed, thus the side surface of the Al layer may corrode. Inaddition, an Au layer may be unintentionally formed on the side surfaceof the Al layer, and an AuAl intermetallic that is highly resistive andfragile may be formed by an interdiffusion of Al and Au.

On the other hand, publicly known document 6 suggests a semiconductorlight-emitting device having an electrode with a structure in which thetop surface and side surface of an Al layer are covered with a W layerand the top surface and side surface of the W layer are further coveredwith an Au. However, in the electrodes of the semiconductorlight-emitting devices, an Au layer is present not only in the upperside of the Al layer but also in the side of the Al layer, therefore,the portion where the Al layer and the Au layer are adjacent to eachother is increased, and the possibility to generate interdiffusion of Aland Au is increased. In addition, in order to prepare an electrode ofthis semiconductor light-emitting device, multiple times ofphotolithography step and multiple times of film formation step arerequired, and the cost is increased as the manufacturing process is morecomplicated, thus it is not practical.

SUMMARY OF THE INVENTION

In view of the above-described problems, an object of the presentinvention is to provide a semiconductor light-emitting device having anelectrode that can be manufactured by a simple method and is unlikely todeteriorate, and a method for forming the electrode.

In order to accomplish the above-described object, the present inventionprovides a semiconductor light-emitting device comprising:

a semiconductor layered structure having a light-emitting layer thatemits light by supplying electric power and

an electrode formed on the semiconductor layered structure, wherein

the electrode comprises:

a reflection layer that reflects light exiting from the light-emittinglayer,

a barrier layer formed on the upper side and side surface of thereflection layer, and

a pad layer formed only on the top surface of the barrier layer.

According to this semiconductor light-emitting device, the structure isformed such that the side surface of a reflection layer is covered by abarrier layer, and also a pad layer is not formed on the side of thereflection layer. Therefore, it is possible to suppress the corrosion ofthe reflection layer and also suppress the interdiffusion of thematerial forming the reflection layer and the material forming the padlayer.

Furthermore, in the semiconductor light-emitting device of theabove-described features, it is preferred that the reflection layer bemade of Al and the pad layer be made of Au.

According to this semiconductor light-emitting device, it is possible tosuppress the corrosion of the reflection layer made of Al and alsosuppress the formation of an AuAl intermetallic formed by theinterdiffusion of the Al forming the reflection layer and the Au formingthe pad layer.

Furthermore, in the semiconductor light-emitting device of theabove-described features, it is preferred that the reflection layer havea film thickness of 40 nm or more and 70 nm or less.

According to this semiconductor light-emitting device, it is possible tosecure enough reflectance and also suppress the generation of a void inthe reflection layer made of Al or suppress the formation of an AuAlintermetallic formed by the interdiffusion of the Al forming thereflection layer and the Au forming the pad layer.

Furthermore, in the semiconductor light-emitting device of theabove-described features, it is preferred that the barrier layer be madeof a high melting point metal having a higher melting point than thoseof Al and Au.

Specifically, it is preferred that the barrier layer contain at leastone of Pt, Mo and W.

According to this semiconductor light-emitting device, it is possible tosuitably suppress the deterioration of the electrode.

Furthermore, in the semiconductor light-emitting device of theabove-described features, it is preferred that the barrier layer have afilm thickness of 200 nm or more.

According to this semiconductor light-emitting device, it is possible toobtain with high certainty an electrode with a structure in which thepad layer is not present in the side of the reflection layer.

Furthermore, in the semiconductor light-emitting device of theabove-described features, it is preferred that the barrier layer have afilm thickness of 300 nm or less.

According to this semiconductor light-emitting device, it is possible tosuppress the formation of the barrier layer unnecessarily thick.

Furthermore, in the semiconductor light-emitting device of theabove-described features, it is preferred that the barrier layer formedon the side surface of the reflection layer have a film thickness of 20nm or more.

According to this semiconductor light-emitting device, the barrier layerhaving enough thickness is formed on the side surface of the reflectionlayer. Therefore, it is possible to suitably suppress the deteriorationof the electrode.

Furthermore, in the semiconductor light-emitting device of theabove-described features, it is preferred that the electrode furtherhave an adhesion layer that contacts with the top surface of thesemiconductor layered structure, and

the reflection layer be formed on the top surface of the adhesion layer.

According to this semiconductor light-emitting device, it is possible tosuitably bring the electrode into contact (e.g., ohmic contact) with thesemiconductor layered structure by providing adhesion layer.

Furthermore, in the semiconductor light-emitting device of theabove-described features, it is preferred that the adhesion layer bemade of Ni, and have a film thickness of 4 nm or less.

According to this semiconductor light-emitting device, it is possible tosuitably suppress the reduction in the reflectance of light by providingan adhesion layer of film thickness of 4 nm or less.

Furthermore, in the semiconductor light-emitting device of theabove-described features, it is preferred that the adhesion layer bemade of Ni, and have a film thickness of 2 nm or more.

According to this semiconductor light-emitting device, it is possible toprevent peeling of a part or all of the electrode since the peelstrength of the electrode can be increased. Therefore, it is possible toincrease the yield of the semiconductor light-emitting device and alsosuppress occurrence of failure during use of the semiconductorlight-emitting device.

In addition, the present invention provides a method for forming anelectrode comprising:

a photolithography step of forming an overhang-shaped resist on asemiconductor layered structure having a light-emitting layer that emitslight by supplying electric power;

a reflection layer forming step of forming a reflection layer on asurface on which the resist is formed;

a barrier layer forming step of forming a barrier layer posterior to thereflection layer forming step;

a pad layer forming step of forming a pad layer posterior to the barrierlayer forming step; and

a lift-off step of removing the resist posterior to the pad layerforming step, wherein

a side surface coverage, that is a value obtained by dividing a sidedeposition rate by a deposition rate vertical to a substrate

is 15% or more at the start point of the barrier layer forming step andis 0% at the end point of the barrier layer forming step and in the padlayer forming step.

According to this method for forming an electrode, it is possible toeasily form an electrode with a structure in which a pad layer is notpresent in the side of the reflection layer, using one resist.

Furthermore, in the method for forming an electrode of theabove-described features, it is preferred that the reflection layer, thebarrier layer and the pad layer are formed in the reflection layerforming step, the barrier layer forming step and the pad layer formingstep, respectively, by continuous layer formation.

According to the method for forming an electrode, it is possible torapidly and easily form each layer.

Furthermore, in the method for forming an electrode of theabove-described features, it is preferred that the barrier layer at theend point of the barrier layer forming step have a film thickness of 200nm or more.

According to this method for forming an electrode, it is possible toobtain with high certainty an electrode with a structure in which thepad layer is not present in the side of the reflection layer.

Furthermore, it is preferred that the method for forming an electrode ofthe above-described features further include an adhesion layer formingstep of forming an adhesion layer that contacts with the top surface ofthe semiconductor layered structure, between the resist forming step andthe reflection layer forming step, and

the film deposition rate of the adhesion layer be more than 0 nm/sec and0.05 nm/sec or less.

According to this method for forming an electrode, fluctuation in thefilm thickness can be reduced, thus it is possible to reproduciblyobtain the semiconductor light-emitting device as designed. Furthermore,since the film thickness of the adhesion layer can be equalized, it ispossible to increase adhesion strength of the adhesion layer. Therefore,it is possible to prevent peeling of the electrode, and also reducecontact resistance.

According to the semiconductor light-emitting device and the method forforming an electrode of the above-described features, it is possible tosuppress the deterioration of the electrode and also form the electrodeby a simple method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E are cross sectional diagrams illustrating an example of amethod for manufacturing the semiconductor light-emitting deviceaccording to an embodiment of the present invention.

FIGS. 2A to 2C are cross sectional diagrams illustrating an example of amethod for manufacturing the semiconductor light-emitting deviceaccording to an embodiment of the present invention.

FIG. 3 is a cross sectional diagram illustrating an example of a methodfor manufacturing the semiconductor light-emitting device according toan embodiment of the present invention.

FIG. 4 is a graph showing the relationship between the wavelength oflight and the reflectance at various Al thicknesses.

FIG. 5 is a graph showing the relationship between the film thickness ofthe Al layer and the reflectance when the wavelength of light is 450 nm.

FIG. 6 is a graph showing the relationship between the film thickness ofthe Al layer and the void occupancy in the Al layer.

FIG. 7 is a graph showing the relationship between the film thickness ofthe Al layer and the rate of an AuAl intermetallic formation.

FIG. 8 is a graph showing the relationship between the film thickness ofthe first Ni layer and the reflectance when the wavelength of light is450 nm.

FIG. 9 is a graph showing the relationship between the film thickness ofthe first Ni layer and the peel strength of the electrode.

FIG. 10 is a graph showing the relationship between the deposition rateof the first Ni layer and the batch to batch variation of thickness.

FIGS. 11A and 11B are a graph showing the relationship between thecumulative layer thickness and the side surface film thickness and aschematic diagram describing a side surface coverage.

FIG. 12 is a graph showing the relationship between the film thicknessof the Pt layer and the side surface film thickness of the Au layer.

FIG. 13 is a graph showing each of operation examples of practicalexamples and comparative examples.

FIG. 14 is a graph showing the relationship between the elapsed time andthe void occupancy in the Al layer, under the conditions of practicaluse of examples.

FIG. 15 is a graph showing the relationship between the elapsed time andthe rate of formation of an AuAl intermetallic, under the conditions ofpractical use of examples.

DETAILED DESCRIPTION OF THE INVENTION

Hereinbelow, as the embodiment of the present invention, a case wherethe present invention is applied to LED is exemplified. However, thesemiconductor light-emitting device to which the present invention canbe applied is not limited to LED. It is possible to apply the presentinvention to the semiconductor light-emitting device in general, thatemits light by supplying electric power via an electrode, for example,laser diode and the like.

<Semiconductor Light-Emitting Device>

First, the semiconductor light-emitting device according to anembodiment of the present invention and an example of the manufacturingmethod thereof are described in reference to FIGS. 1A to 1E, FIGS. 2A to2C, and FIG. 3. FIGS. 1A to 1E, FIGS. 2A to 2C, and FIG. 3 are crosssectional diagrams illustrating an example of a method for manufacturingthe semiconductor light-emitting device according to an embodiment ofthe present invention. FIGS. 1A to 1E exemplify a method for laminatingvarious semiconductor layers (method for forming a semiconductor layeredstructure). In addition, FIGS. 2A to 2C show the steps after FIGS. 1A to1E, and exemplify a method for forming the electrode. In addition, FIG.3 shows the step after FIGS. 2A to 2C, and exemplifies a method forforming a passivation layer.

First, as shown in FIG. 1A, a substrate 10 made of sapphire or the likeis prepared. Then, as shown in FIG. 1B, a main surface that is one sideof the substrate 10 (hereinafter, referred to as surface), is formedinto a concavo-convex shape. For example, the concavo-convex shape canbe formed by forming a resist on the surface of the substrate 10 exceptfor the parts where concave portions (groove) should be formed andcarrying out etching such as ICP (Inductively Coupled Plasma) usinghalogen gas (e.g., a mixed gas of BCl₃, Cl₂ and Ar).

Next, as shown in FIG. 1C, on the surface of the concavo-convex shapedsubstrate 10, an n-clad layer 11 made of n-type GaN, a light-emittinglayer (active layer) 12 having a multiquantum well structure in whichbarrier layers made of GaN and well layers made of In_(x)Ga_(1-x)N(0<x≦1) are alternately laminated and also the first and last layers arebarrier layers, and a p-clad layer 13 made of p-type GaN are laminatedin this order.

The n-clad layer 11, the light-emitting layer 12 and the p-clad layer 13can be laminated, for example, by MOCVD (Metal Organic Chemical VaporDeposition) or the like. In addition, as a dopant of n-type GaN, forexample, Si can be used. Also, as a dopant of p-type GaN, for example,Mg can be used. Moreover, after laminating this p-clad layer 13,annealing may be performed to activate p-type dopant. Also, otherelement such as Al may be contained in the GaN and In_(x)Ga_(1-x)Nconstituting the n-clad layer 11, the light-emitting layer 12 and thep-clad layer 13.

Next, as shown in FIG. 1D, a transparent electrode 14 made of ITO(Indium Tin Oxide) is formed on the p-clad layer 13. This transparentelectrode 14 can be formed, for example, by sputtering.

In the method for manufacturing a semiconductor light-emitting device ofthis example, since an electrode is formed in the n-clad layer 11 in thelater step, it is necessary to expose the n-clad layer 11 in the regionin which the electrode is to be formed. Therefore, as shown in FIG. 1E,a part of the transparent electrode 14, the p-clad layer 13, thelight-emitting layer 12 and the n-clad layer 11 in the region isremoved.

For example, the transparent electrode 14 can be removed by etching withaqua regia or the like. For further example, the p-clad layer 13, thelight-emitting layer 12 and the n-clad layer 11 can be removed byetching such as ICP using halogen gas (e.g., SiCl₄). However, when theetching is performed, it is necessary to form a resist except for thepart to be removed. Here, each resist used in each etching is removedafter the completion of each etching.

Next, a method for forming the electrode is described. Here, forsimplification of description, only a method for forming a p-electrodeis exemplified, but the same shall be applied to a method for forming ann-electrode. However, the p-electrode is provided in a part of the topsurface of the transparent electrode 14, and the n-electrode is providedin a part of the top surface of the above-described n-clad layer 11exposed by etching.

First, as shown in FIG. 2A, an overhang shaped-resist R is formed on thetop surface of the transparent electrode 14. Here, while not shown inFIG. 2A, an overhang shaped-resist R is also formed on the top surfaceof the above-described n-clad layer 11 exposed by etching.

As the method for forming an overhang shaped-resist R, any well-knownforming method may be used. For example, an overhang shaped-resist R maybe formed by forming an ordinary resist of which wall surface of anopening is vertical and then selectively contracting or dissolving thetop surface side of the transparent electrode 14 and the top surfaceside of the n-clad layer 11. In addition, for example, an overhangshaped-resist R may be formed by forming a resist without an opening andthen selectively dissolving it so that the resist has an overhang shape.Here, as shown in FIG. 2A, it is preferable to make the periphery of anopening RO of the resist R concave since the material of the electrodecan be efficiently introduced into the opening RO.

Next, as shown in FIG. 2B, on the top surface of the transparentelectrode 14 on which the resist R is formed and the top surface of then-clad layer 11, various layers constituting the electrode aresequentially formed by electron beam vapor deposition or the like.Specifically, a first Ni layer 2 a (adhesion layer) made of Ni isformed, then an Al layer 2 b (reflection layer) made of Al is formed,then a second Ni layer 2 c made of Ni is formed, then a Pt layer 2 d(barrier layer) made of Pt is formed, then an Au 2 e (pad layer) made ofAu is formed, and a third Ni layer 2 f made of Ni is formed.

Each of the layers 2 a to 2 f is formed on the top surface of the resistR, and the material forming each of the layers 2 a to 2 f enters intothe opening RO of the resist R, thereby forming layers also on each ofthe top surface of the transparent electrode 14 and the top surface ofthe n-clad layer 11. Specifically, on each of the top surface of thetransparent electrode 14 and the top surface of the n-clad layer 11,each of the layers 2 a to 2 f is formed in the order of the first Nilayer 2 a, the Al layer 2 b, the second Ni layer 2 c, the Pt layer 2 d,the Au layer 2 e, and the third Ni layer 2 f, upward from the topsurface of the transparent electrode 14 and the top surface of then-clad layer 11. Here, it is preferred that each of the layers 2 a to 2f be formed by continuous layer formation since it is possible torapidly and easily form a layer.

In each of the top surface of the transparent electrode 14 and the topsurface of the n-clad layer 11, the Pt layer 2 d is formed on the topsurface of the second Ni layer 2 c, and also formed on the side surfaceof the Al layer 2 b. On the other hand, the Au layer 2 e is formed onlyon the top surface of the Pt layer 2 d. Here, a part of the Pt layer 2 dmay be formed not only on the side surface of the Al layer 2 b, but alsoon the side surfaces of the first Ni layer 2 a and the second Ni layer 2c.

The detail of the conditions for forming the above-described layer isset forth below, and it is preferred that each of the layers 2 a to 2 fbe made so as to have film thicknesses of 4 nm for the first Ni layer 2a, 50 nm for the Al layer 2 b, 40 nm for the second Ni layer 2 c, 250 nmfor the Pt layer 2 d, 700 nm for the Au layer 2 e, and 20 nm for thethird Ni layer 2 f.

The first Ni layer 2 a forms ohmic contacts with each of the transparentelectrode 14 and the n-clad layer 11. In addition, the Al layer 2 breflects at least a part of light exiting from the light-emitting layer12. Also, the second Ni layer 2 c rigidly binds the Al layer 2 b and thePt layer 2 d. Moreover, the Pt layer 2 d secures the clearance betweenthe Al layer 2 b and the Au layer 2 e and prevents the interdiffusion ofAl and Au. Also, the Au layer 2 e and the third Ni layer 2 felectrically and mechanically connect to a wire for electricallyconnecting to an external power supply that supplies electric power, orthe like.

Moreover, as shown in FIG. 2C, the resist R on which each of the layers2 a to 2 f are formed is removed (lift-off) by wet etching or the like.Here, while it has been described illustrating an example of the methodfor forming a layer of each of the layers 2 a to 2 f simultaneously oneach of the top surface of the transparent electrode 14 and the topsurface of the n-clad layer 11, it is possible to separately formlayers. In addition, heat treatment may be carried out after removingthe resist R, as necessary.

Next, as shown in FIG. 3, a passivation layer 30 made of SiO₂ is eachformed on the top surface of the transparent electrode 14, the topsurface of the n-clad layer 11, and a part of the side surface and topsurface of each of the layers 2 a to 2 f. The passivation layer 30 isformed, for example, by thoroughly forming SiO₂ by plasma CVD or thelike, thereafter, forming a resist except for a part of the top face ofeach of the layers 2 a to 2 f (part to which wire or the like isbonded), and removing SiO₂ in the part using an etchant such ashydrofluoric acid. Furthermore, at this time, etching is performed untilthe Au layer 2 e is exposed. As a result, an n-electrode 21 is formed onthe top surface of the transparent electrode 14, and a p-electrode 22 isformed on the top surface of the n-clad layer 11. Here, the resist usedin etching is removed after completion of the etching.

According to the above-described steps, a semiconductor light-emittingdevice 1 is formed. However, the semiconductor light-emitting device 1is in the state of wafer (state where the substrate 10 and the n-cladlayer 11 are the same, and a plurality of semiconductor light-emittingdevices 1 is integrated) at this stage, and thus is divided into chipsas necessary. This breaking is performed using the semiconductorlight-emitting device 1 as a unit, and at least one semiconductorlight-emitting device 1 is contained in one chip.

In this breaking step, first, a separation trench is formed on then-clad layer 11. This separation trench can be formed, for example, byforming a resist except for the parts where a separation trench shouldbe formed and performing etching such as ICP using halogen gas (e.g.,SiCl₄) or the like. Here, the resist used in etching is removed aftercompletion of the etching.

Next, the face of the substrate 10 on which concavity and convexity isnot formed (hereinafter, referred to as back surface) is made thin byabrasion or the like. Moreover, a scribe groove is formed on the backsurface of the substrate 10 by laser scribe or the like. At this time,the scribe groove in the substrate 10 is formed in the position oppositeto the separation trench in the n-clad layer 11. Then, for example, ablade is pressed against the scribe groove formed on the back surface ofthe substrate 10, whereby the scribe groove and the separation trenchare split up, so that the wafer is divided.

The chipped semiconductor light-emitting device 1 (hereinafter, referredto as chips), that is produced by the division described above, ismounted, for example, by wire bonding mounting. At this time, the lightexiting from the light-emitting layer 12 is taken out from the topsurface and side surface of the chips to the exterior.

Some of the light exiting from the light-emitting layer 12 is directlyemitted to the outside, and some enter the p-electrode 21 and then-electrode 22. However, the light entering the p-electrode 21 and then-electrode 22 is reflected in the Al layer 2 b in the p-electrode 21and the n-electrode 22, and further reflected in the surface of theconcavo-convex substrate 10, whereby the light can be taken out from thetop surface and side surface of the chips to the exterior. Therefore, itis possible to efficiently exit light from the chips.

As described above, according to the semiconductor light-emitting device1 and the method for forming an electrode of the present embodiment, thestructure is formed such that the side surface of the Al layer 2 b iscovered with the Pt layer 2 d, and also the Au layer 2 e is not formedon the side of the Al layer 2 b. Therefore, it is possible to suppressthe corrosion of the Al layer 2 b and also suppress the formation of anAuAl intermetallic by the interdiffusion of the Al forming the Al layer2 b and the Au forming the Au layer 2 e. In addition, it is possible toeasily form the electrodes 21 and 22 with a structure in which the Aulayer 2 e is not present in the side of the Al layer 2 b using oneresist R. Therefore, it is possible to suppress the deterioration of theelectrodes 21 and 22 and form the electrodes 21 and 22 by a simplemethod.

<Preferred Conditions of Each Layer Forming Electrode>

Next, preferred conditions of each of the layers 2 a to 2 f forming theelectrodes 21 and 22 are described with reference to the drawings.

First, film thickness conditions of the Al layer 2 b for securing enoughreflectance are described with reference to FIG. 4 and FIG. 5. FIG. 4 isa graph showing the relationship between the wavelength of light and thereflectance at various Al film thickness. FIG. 5 is a graph showing therelationship between the film thickness of the Al layer and thereflectance when the wavelength of light is 450 nm. Here, in the graphof FIG. 4, the axis of ordinate is the reflectance (%), and the axis ofabscissa is the wavelength of light (nm). In addition, in the graph ofFIG. 5, the axis of ordinate is the reflectance (%), and the axis ofabscissa is the film thickness of the Al layer 2 b (nm).

As shown in FIG. 4, in the approximate wavelength range of the lightpossibly exiting from the above-described semiconductor light-emittingdevice 1, when the film thickness of the Al layer 2 b is reduced, thereflectance is lowered. Specifically, as shown in FIG. 5, when the Allayer 2 b has a film thickness of less than 40 nm, the reflectance israpidly lowered. On the other hand, when the Al layer 2 b has a filmthickness of 40 nm or more, the reflectance is stably increased.Therefore, it is preferred that the Al layer 2 b have a film thicknessof 40 nm or more including production tolerance.

Next, film thickness conditions of the Al layer 2 b for suppressing thedeterioration of the electrodes 21 and 22 are described with referenceto FIG. 6 and FIG. 7. FIG. 6 is a graph showing the relationship betweenthe film thickness of the Al layer and the void occupancy in the Allayer. FIG. 7 is a graph showing the relationship between the filmthickness of the Al layer and the rate of an AuAl intermetallicformation. Here, in the graph of FIG. 6, the axis of ordinate is thevoid occupancy per unit length of the Al layer 2 b (%), and the axis ofabscissa is the film thickness of the Al layer 2 b (nm). In addition, inthe graph of FIG. 7, the axis of ordinate is the rate of an AuAlintermetallic formation per unit length of the Al layer 2 b (%), and theaxis of abscissa is the film thickness of the Al layer 2 b (nm).

The inventors of the present application have carried out across-sectional SEM (Scanning Electron Microscope) observation for eachsample obtained by heating the electrodes 21 and 22 for various periodsof time at various temperatures (250 to 450° C.) higher than theguarantee temperature of LED (140 to 150° C.), and obtained the voidoccupancy that is the length of the void per unit length of the Al layer2 b. Then, from this observation result, a knowledge that the larger thefilm thickness of the Al layer 2 b and the higher the heatingtemperature, the larger the void occupancy, has been obtained.

Specifically, the inventors of the present application obtained therelationship between the film thickness of the Al layer 2 b and theactivation energy of the void growth, from the Arrhenius plot of thisobservation result. As a result, the activation energy at a filmthickness of 50 nm was 0.41 eV, the activation energy at a filmthickness of 75 nm was 0.32 eV, and the activation energy at a filmthickness of 100 nm was 0.25 eV. Specifically, it was found that thesmaller the film thickness of the Al layer 2 b, the larger theactivation energy of the void growth, and a void is less likely to beformed.

The void occupancy modeled based on the above result is represented by afollowing formula (1). In the following formula (1), S is a voidoccupancy, S₀ is an initial void occupancy, C_(s) is a constant, Ea_(s)is an activation energy of the void growth, k is a Boltzmann constant, Tis an absolute temperature, and t is a heating time.S=S ₀ +C _(s)·EXP(−Ea _(s) /kT)·ln(t)  (1)

FIG. 6 is a graph obtained by applying the formula (1) to the conditionsof practical use of LED. Specifically, in the formula (1), absolutetemperature T is set to 418 K (145° C.), and heating time t is set toone hundred thousand hours, and the void occupancies S at various filmthicknesses of the Al layer 2 b are obtained, and are graphically shown.

As shown in FIG. 6, when the film thickness of the Al layer 2 b isincreased too much, the void occupancy in the Al layer 2 b is increased.Specifically, when the Al layer 2 b has a film thickness of more than 70nm, the void occupancy in the Al layer 2 b is rapidly increased. On theother hand, when the Al layer 2 b has a film thickness of 70 nm or less,the void occupancy is stably reduced. Therefore, from the viewpoint ofsuitably suppressing the generation of a void, it is preferred that theAl layer 2 b have a film thickness of 70 nm or less.

Similarly, the inventors of the present application have carried out across-sectional SEM observation for each sample obtained by heating theelectrodes 21 and 22 for various periods of time at various temperatures(250 to 450° C.) higher than the guarantee temperature of LED (140 to150° C.), and obtained the rate of an AuAl intermetallic that is thelength of the AuAl intermetallic per unit length of the Al layer 2 b.Then, from this observation result, a knowledge that the larger the filmthickness of the Al layer 2 b and the higher the heating temperature,the larger the rate of an AuAl intermetallic has been obtained.

Specifically, the inventors of the present application obtained therelationship between the film thickness of the Al layer 2 b and theactivation energy of the AuAl intermetallic growth, from the Arrheniusplot of this observation result. As a result, the activation energy at afilm thickness of 50 nm was 1.45 eV, the activation energy at a filmthickness of 100 nm was 1.22 eV, and the activation energy at a filmthickness of 200 nm was 0.97 eV. Specifically, it has been found thatthe smaller the film thickness of the Al layer 2 b, the larger theactivation energy of the AuAl intermetallic growth, and the AuAlintermetallic is less likely to be formed (the interdiffusion of Au andAl is suppressed).

The rate of an AuAl intermetallic modeled based on the above result isrepresented by a following formula (2). In the following formula (2), Xis a rate of an AuAl intermetallic, C_(x) is a constant, Ea_(x) is anactivation energy of the AuAl layer growth, k is a Boltzmann constant, Tis an absolute temperature, and t is a heating time.X=C _(x)·EXP(−Ea _(x) /kT)·ln(t)  (2)

FIG. 7 is a graph obtained by applying the formula (2) to the conditionsof practical use of LED. Specifically, in the formula (2), absolutetemperature T is set to 418 K (145° C.), and heating time t is set toone hundred thousand hours, and the rates of an AuAl intermetallic X atvarious film thicknesses of the Al layer 2 b are obtained, and aregraphically shown.

As shown in FIG. 7, when the film thickness of the Al layer 2 b isincreased too much, the rate of an AuAl intermetallic is increased.Specifically, when the Al layer 2 b has a film thickness of more than150 nm, the rate of an AuAl intermetallic is rapidly increased. On theother hand, when the Al layer 2 b has a film thickness of 150 nm, orless, the rate of an AuAl intermetallic is stably reduced. Therefore,from the viewpoint of suitably suppressing the formation of an AuAlintermetallic (interdiffusion of Au and Al), it is preferred that the Allayer 2 b have a film thickness of 150 nm or less.

As described above, the Al layer 2 b has a film thickness of 40 nm ormore and 70 nm or less, whereby it is possible to suppress thegeneration of a void in the Al layer 2 b and suppress the formation ofan AuAl intermetallic resulting from the interdiffusion of Al and Au.

Next, film thickness conditions of the first Ni layer 2 a are describedwith reference to FIG. 8. FIG. 8 is a graph showing the relationshipbetween the film thickness of the first Ni layer and the reflectancewhen the wavelength of light is 450 nm. Here, in the graph of FIG. 8,the axis of ordinate is the reflectance (%), and the axis of abscissa isthe film thickness of the first Ni layer 2 a (nm).

As shown in FIG. 8, when the film thickness of the first Ni layer 2 a isincreased, the reflectance is lowered. Specifically, when the first Nilayer 2 a has a film thickness of more than 4 nm, the reflectance israpidly lowered. On the other hand, when the first Ni layer 2 a has afilm thickness of 4 nm or less, the reflectance is stably increased.Therefore, it is preferred that the first Ni layer 2 a have a filmthickness of 4 nm or less.

As described above, the first Ni layer 2 a has a film thickness of 4 nmor less, whereby it is possible to suitably suppress the lowering of thereflectance of light by providing the first Ni layer 2 a.

Furthermore, film thickness conditions of the first Ni layer 2 a aredescribed with reference to FIG. 9. FIG. 9 is a graph showing therelationship between the film thickness of the first Ni layer and thepeel strength of the electrode. Here, in the graph of FIG. 9, the axisof ordinate is the peel strength of the electrodes 21 and 22 (gF: gramforce), and the axis of abscissa is the film thickness of the first Nilayer 2 a (nm). Here, the peel strength refers to the force required topeel an object (electrodes 21 and 22 in this case) from the bonded face.In addition, in FIG. 9, the unit of the peel strength of the electrodes21 and 22 is gF (gram force), and 1 gF≈9.8×10⁻³N.

As shown in FIG. 9, when the first Ni layer 2 a has a film thickness ofless than 2 nm, the peel strength of the electrodes 21 and 22 is rapidlyreduced. On the other hand, when the first Ni layer 2 a has a filmthickness of 2 nm or more, the peel strength of the electrodes 21 and 22is stably increased. Therefore, it is preferred that the first Ni layer2 a have a film thickness of 2 nm or more.

As described above, the first Ni layer 2 a has a film thickness of 2 nmor more, whereby the peel strength of the electrodes 21 and 22 can beincreased, then it is possible to prevent peeling of a part or all ofthe electrodes 21 and 22.

Therefore, it is possible to increase the yield of the semiconductorlight-emitting device 1 and also suppress the occurrence of failureduring use of the semiconductor light-emitting device 1.

Next, film forming conditions of the first Ni layer 2 a are describedwith reference to FIG. 10. FIG. 10 is a graph showing the relationshipbetween the film deposition rate of the first Ni layer and the batch tobatch film thickness variation. Here, in the graph of FIG. 10, the axisof ordinate expresses the batch to batch variation of film thickness ofthe first Ni layer 2 a by 3σ (nm), and the axis of abscissa is the filmdeposition rate of the first Ni layer 2 a (nm/sec). Here, 3σ refers tothree times of the standard deviation, and almost all data (filmthickness) belongs to the range of average value±3σ (when the variationis normally-distributed, 99.7% of data (film thickness) belongs to thisrange). Therefore, as 3ρ is smaller, whole variation of data (filmthickness) is small, and the data (film thickness) is gathered around apredetermined value.

As shown in FIG. 10, when the film deposition rate of the first Ni layer2 a is more than 0.05 nm/sec, the batch to batch variation of filmthickness 3σ is rapidly increased. On the other hand, when the filmdeposition rate of the first Ni layer 2 a is 0.05 nm/sec or less, thebatch to batch variation of film thickness 3σ is stably reduced (inexample of FIG. 10, nearly zero). Therefore, it is preferred that thefilm forming rate of the first Ni layer 2 a be 0.05 nm/sec or less.Here, the film deposition rate of the first Ni layer 2 a is naturallymore than 0 nm/sec.

As described above, the film deposition rate of the first Ni layer 2 ais 0.05 nm/sec or less, whereby the variation in the film thickness canbe reduced, thus it is possible to reproducibly obtain the semiconductorlight-emitting device 1 as designed. Furthermore, since the filmthickness of the first Ni layer 2 a can be equalized, it is possible toincrease adhesion of the first Ni layer 2 a. Therefore, it is possibleto prevent peeling of the electrodes 21 and 22, and also reduce thecontact resistance.

Next, film thickness conditions of the Pt layer 2 d are described withreference to FIGS. 11A and 11B and FIG. 12. Hereinbelow, for the sake ofconvenience of the description, the film thickness in the direction ofthe top surface is referred to as “film thickness” as heretofore, andthe film thickness in the direction of the side surface (specifically,the film thickness in the direction of the side surface of the Al layer2 b) is referred to as “side surface film thickness”, to distinguish.

FIG. 11A is a graph showing the relationship between the cumulativevalue of the film thickness (cumulative film thickness) and the sidesurface film thickness when the first Ni layer 2 a, the Al layer 2 b,the second Ni layer 2 c, the Pt layer 2 d and the Au layer 2 e areformed in order (continuous film formation), and FIG. 11B is a schematicdiagram describing a side surface coverage. Here, in the solid linegraph in FIG. 11A, the axis of ordinate is the side surface filmthickness of the Pt layer 2 d and the Au layer 2 e (nm), and the axis ofabscissa is the cumulative film thickness (nm). In addition, in thedashed-dotted line graph in FIG. 11A, the axis of ordinate is the sidesurface coverage (%) of the Pt layer 2 d and the Au layer 2 e, and theaxis of abscissa is the cumulative film thickness (nm). Also, the graphsshown in FIG. 11A illustrate a case where the first Ni layer 2 a has afilm thickness of 4 nm, the Al layer 2 b has a film thickness of 50 nm,and the second Ni layer 2 c has a film thickness of 40 nm.

In addition, FIG. 12 is a graph showing the relationship between thefilm thickness of the Pt layer and the side surface film thickness ofthe Au layer. Here, in the graph of FIG. 12, the axis of ordinate is theside surface film thickness of the Au layer 2 e, and the axis ofabscissa is the film thickness of the Pt layer 2 d.

As shown in the solid line graph in FIG. 11A, the side surface filmthickness of the Pt layer 2 d formed on the side surface of the Al layer2 b is increased until having a film thickness of 200 nm, and in a filmthickness of 200 nm or more, the side surface film thickness is constantat the value of 20 nm or more. Specifically, when the Pt layer 2 d has afilm thickness of 200 nm or more, the Pt layer 2 d and the Au layer 2 eare not formed on the side surface of the Al layer 2 b after that, andthe Pt layer 2 d and the Au layer 2 e are formed only to the upperdirection.

The similar thing is described based on the side surface coverage. Here,as shown in FIG. 11B, the side surface coverage is the value obtained bydividing the film deposition rate of the side surface film thickness(increment) T_(L) by the film deposition rate of the film thickness(increment) T_(v) (T_(L)/T_(v)).

As shown in the dashed-dotted line graph in FIG. 11A, as the formationof the Pt layer 2 d and the Au layer 2 e are progressed to increase thetotal film thickness, the side surface coverage that is 15% or more atthe start of film deposition is gradually reduced. Specifically, as thefilm thickness of the Pt layer 2 d is increased, the Pt layer 2 d andthe Au layer 2 e are less likely to be formed on the side surface of theAl layer 2 b. Moreover, when the Pt layer 2 d has a film thickness of200 nm or more, the side surface coverage is 0%. Specifically, when thePt layer 2 d has a film thickness of 200 nm or more, the Pt layer 2 dand the Au layer 2 e are not formed on the side surface of the Al layer2 b.

As shown in FIG. 12, when the Pt layer 2 d is made so as to have a filmthickness of less than 200 nm, the side surface film thickness of the Aulayer 2 e is more than 0 nm (the Au layer 2 e is formed in the directionof the side surface of the Al layer 2 b). On the other hand, when the Ptlayer 2 d is made so as to have a film thickness of 200 nm or more, theside surface film thickness of the Au layer 2 e is 0 nm (the Au layer 2e is not formed in the direction of the side surface of the Al layer 2b).

Therefore, the Pt layer 2 d is made so as to have a film thickness of200 nm or more using this property, whereby it is possible to preventthe deposition of the Au layer 2 e in the side of the Al layer 2 b inthe film deposition stage of the Au layer 2 e.

On the other hand, it is preferred that the Pt layer 2 d be made to havea film thickness of 200 nm or more that is the above-described lowerlimit, considering product variation and the like. However, it is notpreferred that the film thickness of the Pt layer 2 d be excessivelyincreased since the amount of necessary source materials is increased,or the time required for manufacturing process gets longer. Therefore,it is preferred that the Pt layer 2 d have a film thickness of, forexample, 300 nm or less. Specifically, for example, assuming the productvariation is ±50 nm or so, it is preferred that the Pt layer 2 d be setto have a film thickness of 250 nm.

As described above, it is preferred that the Pt layer 2 d have a filmthickness of 200 nm or more and 300 nm or less. At this time, it ispossible to obtain with high certainty the electrodes 21 and 22 with astructure in which the Au layer 2 e is not present in the side of the Allayer 2 b. In addition, it is possible to form the Pt layer 2 d havingenough side surface film thickness (20 nm or more) capable of suitablysuppressing the deterioration of the electrodes 21 and 22. Furthermore,it is possible to suppress the formation of the Pt layer 2 dunnecessarily thick.

Next, operation examples of the semiconductor light-emitting device 1satisfying the above conditions (hereinafter, referred to as examples)and operation examples of the semiconductor light-emitting device notsatisfying the above conditions and not having a reflection layer(hereinafter, referred to as comparative examples) are each describedwith reference to FIG. 13. FIG. 13 is a graph showing each of theoperation examples of examples and comparative examples.

The graph of FIG. 13 shows the result measured by supplying apredetermined electric current (e.g., 85 mA) to a semiconductorlight-emitting device and focusing exit light by an integrating sphereas an optical output power by wavelength. Here, in FIG. 13, theoperation results of examples are denoted by ♦ in the figure, and theoperation results of comparative examples are denoted by □ in thefigure. Here, in the graph of FIG. 13, the axis of ordinate is theoptical output power (mW), and the axis of abscissa is the wavelength(nm).

As shown in FIG. 13, in the whole of emission wavelength, the opticaloutput of the light exiting from examples is larger than the opticaloutput of the light exiting from comparative examples (in this case, 3.8mW or so, and 4% or so in the ratio). Specifically, examples can exitlight more efficiently than comparative examples.

In addition, high temperature high humidity bias test (temperature: 85°C., humidity: 85%, reverse bias: −5 V, drive time: 1000 hours) has beenperformed on examples, and it has been able to confirm that there is noproblem for forward voltage properties, optical output power properties,and breakdown voltage properties and also confirm that there is noproblem for corrosion.

Also, the incidence of void formation and the rate of an AuAlintermetallic of examples under the conditions of practical use(temperature: 145° C.) are described with reference to FIG. 14 and FIG.15. FIG. 14 is a graph showing the relationship between the elapsed timeand the void occupancy in the Al layer, under the conditions ofpractical use of examples. Also, FIG. 15 is a graph showing therelationship between the elapsed time and the rate of an AuAlintermetallic, under the conditions of practical use of examples. Here,the graph of FIG. 14 is obtained by the formula (1) when the Al layer 2b has a film thickness of 70 nm, and the graph of FIG. 15 is obtained bythe formula (2) when the Al layer 2 b has a film thickness of 70 nm. Inaddition, in the graph of FIG. 14, the axis of ordinate is the voidoccupancy per unit length of the Al layer 2 b (%), and the axis ofabscissa is the elapsed time under the conditions of practical use (h).Also, in the graph of FIG. 15, the axis of ordinate is the rate of anAuAl intermetallic per unit length of the Al layer 2 b (%), and the axisof abscissa is the elapsed time under the conditions of practical use(h).

As shown in FIG. 14, the incidence of void formation after a lapse ofone hundred thousand hours is 1.8%. On the other hand, as shown in FIG.15, the rate of an AuAl intermetallic after a lapse of one hundredthousand hours is 8×10⁻⁵%. Both values are in the acceptable level interms of the reliability of a semiconductor light-emitting device.

<Variations>

The constitution of the semiconductor light-emitting device 1 describedabove is merely one example, and may be properly changed. For example,any well-known structure can be adopted to the semiconductor layeredstructure 11 to 14. However, it is preferred that the semiconductorlight-emitting device have a structure having a light-emitting layer andan electrode for supplying electric power to the light-emitting layer.

In addition, the constitution of the p-electrode 21 and the n-electrode22 described above is merely one example, and may be any constitution solong as it has a reflection layer that reflects at least a part of lightexiting from the light-emitting layer 12, a pad layer, a barrier layerthat suppresses reaction of the reflection layer and the pad layer.

Moreover, in the above example, while a case where each of the layers 2a to 2 f constituting the p-electrode 21 and the n-electrode 22 is madeof one type of metal is exemplified, at least one of these layers maycontain plural types of metals. In addition, a part or all of each ofthe layers 2 a to 2 f constituting the p-electrode 21 and then-electrode 22 may be different from the above examples.

For example, while a case where the barrier layer is made of Pt isexemplified, the barrier layer may be made of other materials. However,from the viewpoint of suitably suppressing the deterioration of thep-electrode 21 and the n-electrode 22, it is preferred that the barrierlayer be made of a high melting point metal having a higher meltingpoint than those of the material forming the reflection layer (Al) andthe material forming the pad layer (Au) (for example, containing atleast one of Pt, Mo, and W).

The semiconductor light-emitting device and the method for forming anelectrode according to the present invention have suitably used for LEDor the like, mounted on a lighting system or the like.

Although the present invention has been described in terms of thepreferred embodiment, it will be appreciated that various modificationsand alternations might be made by those skilled in the art withoutdeparting from the spirit and scope of the invention. The inventionshould therefore be measured in terms of the claims which follow.

What is claimed is:
 1. A semiconductor light-emitting device comprising:a semiconductor layered structure having a light-emitting layer thatemits light upon supply of electric power and an electrode formed on thesemiconductor layered structure, wherein the electrode comprises: areflection layer that reflects light exiting from the light-emittinglayer and is made of Al, a barrier layer formed on an upper side and aside surface of the reflection layer, a pad layer that is formed only ona top surface of the barrier layer and is made of Au, wherein thereflection layer has a film thickness of 40 nm or more and 70 nm orless, and a layer that is made of Ni and formed on a top surface of thepad layer.
 2. The semiconductor light-emitting device according to claim1, wherein the barrier layer is made of a high melting point metalhaving a higher melting point than those of Al and Au.
 3. Thesemiconductor light-emitting device according to claim 2, wherein thebarrier layer contains at least one of Pt, Mo and W.
 4. Thesemiconductor light-emitting device according to claim 1, wherein thebarrier layer has a film thickness of 200 nm or more.
 5. Thesemiconductor light-emitting device according to claim 1, wherein thebarrier layer has a film thickness of 300 nm or less.
 6. Thesemiconductor light-emitting device according to claim 1, wherein thebarrier layer formed on the side surface of the reflection layer has afilm thickness of 20 nm or more.
 7. The semiconductor light-emittingdevice according to claim 1, wherein the electrode further comprises anadhesion layer that contacts with a top surface of the semiconductorlayered structure, and the reflection layer is formed on a top surfaceof the adhesion layer.
 8. The semiconductor light-emitting deviceaccording to claim 7, wherein the adhesion layer is made of Ni, and hasa film thickness of 4 nm or less.
 9. The semiconductor light-emittingdevice according to claim 7, wherein the adhesion layer is made of Ni,and has a film thickness of 2 nm or more.
 10. The semiconductorlight-emitting device according to claim 1, wherein the electrodefurther comprises a layer that is made of Ni and formed between thereflection layer and the barrier layer.
 11. A semiconductorlight-emitting device comprising: a semiconductor layered structurehaving a light-emitting layer that emits light upon application ofelectric power and an electrode formed on the semiconductor layeredstructure, wherein the electrode comprises: a reflection layer thatreflects light exiting from the light-emitting layer and is made of Al,a barrier layer formed on an upper side and a side surface of thereflection layer, a pad layer formed only on a top surface of thebarrier layer and is made of Au, wherein the barrier layer formed on theupper side of the reflection layer has a film thickness of 200 nm ormore, and the barrier layer formed on the side surface of the reflectionlayer has a film thickness of 20 nm or more, and a layer that is made ofNi and formed on a top surface of the pad layer.
 12. The semiconductorlight-emitting device according to claim 11, wherein the reflectionlayer has a film thickness of 40 nm or more and 70 nm or less.
 13. Thesemiconductor light-emitting device according to claim 11, wherein thebarrier layer is made of a high melting point metal having a highermelting point than those of Al and Au.
 14. The semiconductorlight-emitting device according to claim 13, wherein the barrier layercontains at least one of Pt, Mo and W.
 15. The semiconductorlight-emitting device according to claim 11, wherein the barrier layerhas a film thickness of 300 nm or less.
 16. The semiconductorlight-emitting device according to of claim 11, wherein the electrodefurther comprises an adhesion layer that contacts with a top surface ofthe semiconductor layered structure, and the reflection layer is formedon a top surface of the adhesion layer.
 17. The semiconductorlight-emitting device according to claim 16, wherein the adhesion layeris made of Ni, and has a film thickness of 4 nm or less.
 18. Thesemiconductor light-emitting device according to claim 16, wherein theadhesion layer is made of Ni, and has a film thickness of 2 nm or more.19. The semiconductor light-emitting device according to claim 11,wherein the electrode further comprises a layer that is made of Ni andformed between the reflection layer and the barrier layer.